Method for changing the characteristics of a ship and a hull form of an icebreaking ship

ABSTRACT

A hull for improved seakeeping characteristics of an icebreaking ship so that the breadth of the ship&#39;s hull will be decreased on the aft side of a wide foreship in the vicinity of the waterline. It can be implemented with solid hull form or so that on the side of the hull there is a section removable with respect to the solid hull, location or position of which will be changed with respect to the hull. In the solid structure on the side of the hull there will be formed a recess and above it a reamer, rising towards the stern and formed by a sloping surface. The recess forms a pass between the bilge keel and the reamer becoming wider towards the stern and joining the hull&#39;s narrower part of the aft body of the ship.

The object of the invention relates to a method for improving theseakeeping characteristics such as the rolling characteristics of anicebreaking ship and a hull form of an icebreaking ship.

One of the problems is shipbuilding is that the hull form of the shipusually is a compromise solution of the required characteristics. If thehull form of the ship is designed to benefit one particular use areaonly, other characteristics may get worse. One of these objects is thebreadth of the ship's hull. There are use areas that require arelatively wide hull. This, however, results in worse seakeepingperformance of this kind of vessel.

The above mentioned problem occurs especially in modern icebreakers.Nowadays the main characteristics of an ice-breaker are, in the firstplace, determined by the breadth of required channel to be formed in theice. Therefore the aim is to make the icebreakers wide in order toachieve the required breadth of the waterway in ice. There are alsocases that the necessity of changing the breadth of the hull can bechanged just the opposite. In particular conditions instead of narrowhull a wide hull would be more advantageous for stability reasons.

Eventual draught restrictions and displacement of the vessel also haveinfluence on the hull form of a modern icebreaker. Furthermore, forimproving the icebreaker capability and manoeuvrability of the vesselthe sides of icebreaker are made sloping and the length-to-breadth ratioof the ship as small as possible. This results in the fact thaticebreaker is always relatively wide.

Due to the above, modern icebreakers have developed into effectivespecial vessels showing their best characteristics especially in thewinter in icebreaking operations. On the other hand, this kind ofspecialization in one use area only has also led into disadvantages.Hull of modern icebreakers is much too wide for open water operations,which makes the seakeeping performance considerably worse in open water.

The poor behaving patterns of icebreakers in open water are caused inthe first place by too big initial stability of a wide hull. Theconsequences of this are increased tendency to roll and the fact thatrolling has very intensive accelerations in beam seas. Furthermore thehull form of an icebreaker is not apt to dampen efficiently the rollingmotion because the side form in an icebreaker is round at bilge.

With bilge keels the seakeeping characteristics of an ordinary sea-goingship can be improved, but in icebreakers they can't be used. Ice causesso much ice loads to the hull of an icebreaker that bilge keels wouldn'twithstand it. Without bilge keels a ship with wide hull rolls heavily onrough seas and therefore, accordingly, accelerations of ship are in beamseas one magnitude bigger than in a ship designed for open water use.Thus on rough seas it is difficult, often even impossible to carry outoperations on an icebreaker. This essentially limits the open water useof an icebreaker.

On the other hand, however, the operating time of icebreakers in winteris so short that it should be able to use them also in summertime inopen water. Suitable tasks would be, for example, towing and supplyoperations. Good behaviour in open water would be needed also in normaloperations of an icebreaker because icebreakers must be able to movelong distances also in open water during transfers. On modernicebreakers these transfers are, however, extremely inconvenient.Accelerations onboard can then be so intensive that it's intolerable forhuman body. So rough seas can make the transfers of an icebreakerimpossible to carry out.

The object of the present invention is to eliminate the above mentionedproblems. It will be achieved by means of a method according to theinvention characterized in that for improving the seakeepingcharacteristics of an icebreaking ship the hull breadth of the ship willbe decreased on the aft side of relatively wide foreship at least inpart of the ship's length so that in this area the hull will be narrowermainly in the vicinity of the water line and possibly also above thewater line but not in lower part of the hull under water, whereupon dueto narrowing of the wide hull there will be a bilge keel formed on bothsides of the hull forming an extension to the foreship.

Correspondingly, it is possible, if necessary, to increase the breadthfor example of an arctic research vessel or supply vessel. Increasedbreadth leads to better stability. Especially regarding to the damagestability there are more requirements for hard conditions.

The object of the present invention is also a hull form of anicebreaking ship. The hull form according to the invention ischaracterized in that on sides of the hull of an icebreaking ship, onthe aft side of relatively wide foreship there has been formed or can beformed at least one such recess or narrowed-off place, where at least apart of ship's hull in this area is narrower than the foreship mainly inthe vicinity of the water line and possibly also above the water linebut not in the lower part of the hull under water, whereupon due tonarrowing of the wide hull after the foreship there will be formed abilge keel on both sides of the hull in this area forming an extensionto the foreship at least in part of the ship's hull.

The structure according to the invention enables for example anicebreaking ship to be formed so that it can be used also in open waterbecause its seakeeping characteristics will be improved. Thus it hasbeen achieved a combined hull form for an icebreaking and for a vesselmeant to be used in open water.

According to the present invention the hull of an icebreaker can benarrowed for open water use so much that the ship's initial stabilitydecreases to the same level as in normal sea-going supply ships. Thismeans that is beam seas the motions of an icebreaker can be achieved aspleasant as of other open sea vessels without having to content withworse hull form in icebreaking operations. Thus the possibilities to usemodern icebreakers for open water operations can be essentiallyimproved.

According to a preferred embodiment the combined icebreaker and ship foropen water use can be achieved so that on sides of the ship's hull thereare side tanks consisting of one section or several sections which arement to be removed in open water operations, the removal of these tanksdecreasing the ship's stability to a level suitable for open wateroperations.

After removing the removable side tank sections there will be in thebilge section of the ship's hull roll dampening projections formed whichat the same time operate as bilge keels of the ship thus dampening theship's rolling. The bilge keels also decrease the heaving and pitchingof the ship. Furthermore, the said side structure enables to keep a wideworking deck though the stability of the ship otherwise corresponds to anarrower ship.

In the main deck of the ship there can be an overhang in the area of theremovable side tanks, thus the ship's cargo deck has a full width.

The overhang of the deck and the bilge projections thus form fenderscorresponding to the double side. Thus by means of the deck overhang andbilge shape there will be on the sides of the ship a protectingstructure which replaces the wide double side required for supplyvessels.

It can be considered that the most important advantage of the presentinvention is the fact that hull form of the ship remains the bestpossible both for icebreaking and open water operations thus makingpossible and profittable to use an icebreaker type ship for example as asupply ship. Also the safety of the crew will be improved during openwater operations.

The removable tank sections can be attached to the ship's hull in manydifferent ways. The attachment can be done, for example, by means ofmechanical fixing means, by hydraulic means or by partly welding theremovable side sections into the ship's hull. In practice, the strengthof the joints will also be increased by the fact that in winter duringicebreaking operations the water will freeze between the removable tanksections and hull thus acting as an equalizer for the local stressesdirected to the tanks and joints.

The transportation of the removable tank sections onto ship's side andout of it can be carried out alternatively by means of a crane, byfloating using ballast in tanks or by means of an attaching devicedesigned especially for that purpose. The removable tank sections canalso be maintained and repaired in suitable conditions ashore withoutdisturbing the ship's operations, because during the open water seasonthe ship operates without removable tank sections.

According to another preferred embodiment the ship's hull form ischaracterized in that on the both sides of the hull there has beenformed at least one such recess or narrowed-off place which makes atleast a part of the ship's hull narrower at least in the area of thewater line thus improving the seakeeping characteristics of the ship.

The structure according to the invention enables achieving, for example,an icebreaking vessel which is possible to use also in open wateroperations. For this purpose on the both sides of an icebreaker's hullthere has been formed at least one recess, thus decreasing the breadthof ship's hull at least in the area of the water line and improving theseakeeping characteristics of the ship in open water operations.

The invention establishes a combined hull form of an ice-breaker andsea-going ship for open water operations.

According to the present invention the hull of an icebreaker can be madeso much narrower for open water operations that the initial stability ofthe ship decreases to the same level as in normal sea-going supplyships. This means that the motions of an icebreaker in beam seas can bemade as good as on other sea-going ships without having to content withworse hull form in icebreaking operations. Thus the possibilities to usemodern icebreakers for open water operations can be essentiallyimproved.

Below the recesses there will be in the bilge section of the ship's hullroll dampening projections formed which at the same time operate asbilge keels of the ship thus dampening the ship's rolling. The bilgekeels also decrease heaving and pitching of the ship. Furthermore, thesaid side structure enables to keep a wide working deck though thestability of the ship otherwise corresponds to a narrower ship.

In the area of recesses there can be an overhang in the main deck, thusthe ship's cargo deck having full width.

The overhang of the deck and the bilge projections thus form fenderscorresponding to the double side. Thus by means of the deck overhang andbilge shape there will be on the sides of the ship a protectingstructure which replaces the wide double side required for supplyvessels.

It can be considered that the most important advantage of the presentinvention is the fact that hull form of the ship remains the bestpossible both for icebreaking and open water operations thus makingpossible and profitable to use an icebreaker type ship for example as asupply ship. Also the safety of the crew will be improved during openwater operations.

According to another preferred embodiment on the ship's hull on the aftside of the wide foreship there has been formed a appendage or reamerconnected to the recess, the lower surface of the reamer being formed bysloping surface that rises up towards aftship and thus forming the uppersurface of the recess. This means that the recess will be preferablyformed between the bilge keel and the reamer so that the recess forms apass between the bilge keel and the reamer, the said pass becoming widertowards the aftship joining the narrower part of the aftship hull. Thiskind of structure makes the ship suitable both for icebreaking and foropen water operations.

The invention is in the following described by the aid of examplesreferring to the attached drawings where

FIG. 1 shows the side view of a hull form of a ship according to anembodiment of the invention.

FIG. 2 shows the top view of the hull form of the ship in FIG. 1.

FIG. 3 shows a section along the line III--III in FIG. 1.

FIG. 4 shows a section along the line IV--IV in FIG. 1.

FIG. 5 corresponds to FIG. 1 and shows a side view of a hull form of aship according to the second embodiment.

FIG. 6 shows the top view of the hull form of the ship in FIG. 5.

FIG. 7 shows a section along the line VII--VII in FIG. 5.

FIG. 8 corresponds to FIG. 1 and shows a side view of a hull form of aship according to the third embodiment.

FIG. 9 shows the top view of a hull form of the ship in FIG. 8.

FIG. 10 shows a section along the line X--X in FIG. 8.

FIG. 11 corresponds to FIG. 1 and shows a side view of a hull form of aship according to the fourth embodiment.

FIG. 12 shows a section along the line XII--XII in FIG. 11.

FIG. 13 shows a section along the line XIII--XIII in FIG. 11.

FIG. 14 corresponds to FIG. 1 and shows schematically a side view of ahull form of a ship according to the fifth embodiment including the sidestructure provided with the removable side sections according to thepresent invention.

FIG. 15 shows the top view of the ship in FIG. 14.

FIG. 16 shows a section along the line XVI--XVI in FIG. 14.

FIG. 17 corresponds to FIG. 16 and shows a cross section of a shipprovided with a different side structure.

FIG. 18 shows a detail of a cross section of a ship's side structurewith a separate removable side section attached.

FIG. 19 corresponds to FIG. 18 and shows a detail of the cross sectionof the ship's side structure with the separate section removed.

FIG. 20 shows schematically the changing of the ship's side structure.

FIG. 21 corresponds to FIG. 18 and shows the second embodiment of adetail of the ship's side structure according to FIG. 17.

FIG. 22 corresponds to FIG. 21 and shows a detail of the ship's sidestructure with the separate section removed.

FIG. 23 corresponds to FIG. 21 and shows a detail of the ship's sidestructure according to the third embodiment.

FIG. 24 corresponds to FIG. 18 and shows a detail of the ship's sidestructure according to the fourth embodiment.

FIG. 25 corresponds to FIG. 24 and shows a detail of the ship's sidestructure with the separate section removed.

FIG. 26 shows a section of the ship's side structure along the lineXXVI--XXVI in FIG. 18.

FIG. 27 shows a section of the ship's side structure along the lineXXVII--XXVII in FIG. 18.

FIG. 28 shows a section of a locking part of the ship's side structurealong the line XXVIII--XXVIII in FIG. 18.

FIG. 29 corresponds to FIG. 21 and shows the fifth embodiment of theside structure.

FIG. 30 corresponds to FIG. 29 with the separate section removed.

FIG. 31 corresponds to FIG. 15 and shows a top view of a ship accordingto the sixth embodiment.

FIG. 32 corresponds to FIG. 29 and shows the sixth embodiment of theside structure.

FIG. 33 corresponds to FIG. 32 with the separate section turned toanother position.

FIG. 34 corresponds to FIG. 32 and shows the seventh embodiment of theside structure.

FIG. 35 corresponds to FIG. 34 with the separate section moved toanother place regarding to the ship's hull.

FIG. 36 corresponds to FIG. 32 and shows the eighth embodiment of theside structure.

FIG. 37 corresponds to FIG. 36 with the separate section moved toanother place regarding to the ship's hull.

In FIG. 1 is shown a side view of a ship's full form 20, where thebreadth of fore body 65 of the hull 21 corresponds to conventionalrelatively wide icebreaker, whereas the aft body 64 is formed clearlynarrower than the fore body 65. Narrowing of the hull 21 has beenimplemented so that in the mid body on both sides of the hull in thevicinity of the water surface 22 there have been formed recesses 60,above and below of which the hull 21, however, still continues in wideshape some distance towards the aft ship. The recess 60 is thus formedbetween the surface 63, 66 and 71.

Surface 63 is mainly vertical surface which forms the narrowing. Surface71 above the recess 60 is the lower surface of the appendage 70 orreamer and surface 66 situating below is the upper surface of the bilgekeel 24. The purpose of the recess 60 and the narrower aft body 64 ofthe hull 21 connected to it is to improve the ship's seakeepingcharacteristics.

Appendage 70 or reamer is meant to break the ice while the ship ismoving backwards. That's why the sloping lower surface 71 of appendage70 has been placed into the area of the water surface 22 i.e. the ice tobe broken so that one part of the reamer 70 is above the water surfaceand another part is below it. The sloping lower surface 71 of the reamer70 is sloping in respect with both the longitudinal axis and transversalaxis of the ship. Lengthwise the angle Φ of the sloping lower surface 71of the reamer 70 in respect with the water surface 22 is, for example,15°20°, in most advantageous case 15°. The bilge keel 24 is formed inthe area of the recess 60 in mid body of the ship's hull 21 as anextension of wide fore body. Slop of its upper surface is about 15° inrespect with the horizontal plane.

FIG. 2 shows the top view of the hull 21 of the ship of FIG. 1. Thefigure shows that the fore body 65 of the hull 21 is wide and aft body64 is narrower. Broken line shows the recesses 60 on the sides of thehull 21, which are staing between the bilge keel 24 and reamer 70. Asshows in the figure the breadth of the hull in the area of the bilgekeel 24 is not quire as broad as in the area of the reamer 70. This dueto the fact that the sides of the hull 21 are sloping. Because the hull21 thus in its lower part is narrower than in its upper part,accordingly the hull is narrower also in the area of the bilge keel 24.

The section presented in FIG. 3 shows how the ship's hull 21 accordingto the invention is narrow in its aft body i.e. in the area of thesection and aft from it. At front of the section or in its fore body theship is wider. Near the narrowed place on both sides of the hull 21there are formed recesses 60 which are situated between the slopingupper surface 66 of the bilge keel 24 situated under the water surface22 and the sloping lower surface 71 of the appendage 70 or reamer. Frontof the recess 70 adjoins on a wedge-shaped, mainly vertical surface 63.

FIG. 3 shows that amidships the outer surfaces of the bilge keel 24 andreamer 70 principally follow the outer dimensions of the ship's hull 21.Because icebreaking ship is concerned, the fore body of the hull must berelatively wide so that a channel of sufficient width would be formed inthe ice but, on the other hand, the sides must also be slightly sloping.That's why the maximum breadth of the bilge keel 24 is smaller than themaximum breadth of the reamer 70. At the same time the bilge keel 24 isalso protected by the side so that the bilge keel won't crash into thequay.

FIG. 3 also shows that during the backing the sloping lower surface 71of icebreaking reamer 70 reaches above the water surface 22 as well asbelow it. This sloping surface 71 has been inclined in respect with boththe longitudinal axis of the ship 20, as can be seen in FIG. 1, and thetransversal axis of the ship. The angle β of the sloping surface 71showed in FIG. 3 in crosswise direction regarding the water surface 22is, for example, between 0°-45°, preferably 30°.

The angle α of the upper surface 66 of the bilge keel 24 regarding thewater surface 22 is not constant. The cross section of FIG. 3 shows thatat its least this angle is, for example, 15°, but in the narrower aftbody of the ship 20 the bilge keel becomes smaller and approaches theship's side.

In FIG. 4 is shown another cross section of the ship 20 furtherillustrating the shape of recesses 60 on the sides of the ship 20. Fromthe figure can be seen that the ship's hull 21 is in fore body quiteconventional bow of an icebreaking ship, but after fore body towards theaft body on the sides of the ship have been formed these recesses 60.Recesses 60 are directed in wedge shape towards the center line of theship all the way to the breadth of the aft body. The recesses arelimited to the upper surface 66 of the bilge keel 24, lower surface 71of the reamer 70 and wedge-shaped narrowing surface 63.

In FIG. 5 is shown the side view of the hull 21 of the ship 20 accordingto the invention and in FIG. 6 is shown the corresponding top view ofthe hull. In this embodiment the fore body 65 of the hull 21 is formedby conventional fore body of an icebreaker. Wide fore body, however,becomes narrower from point 62 towards the aft body. After wedge-shapednarrowing surface 63 the aft body 64 of the hull 21 is above the waterline 22 straight and clearly narrower than the fore body 65.

The narrowing, which begins from point 62 of the hull's 21 fore body 65,however, concerns only the area near the water line 22 of the hull 21and part of the hull above it. Below the water line 22 also in the aftside of the narrowing surface 63 the hull shape has a wide area. Thisshape can clearly be seen in the cross section of the hull 21 in FIG.27. Also in the narrowed place in the lower part of the hull 21 remainsbilge 27, which reaches to breadth of the hull's fore body 65 andfunctions as a bilge keel increasing the stability of the ship 20. Inthis embodiment the hull's side in the area of the bilge keel is mainlyvertical and the upper surface 66 of the bilge keel forms an angle ofabout 15° with the horizontal plane.

It can be see from the FIG. 5 the arched shape of the bilge keel 27 inthe side view. The curve of the outermost edge 24 of the bilgecorresponds to the arched shape of the hull 21 i.e. the shape of thebilge keel 27 corresponds to the direction of the water flow against thehull.

FIG. 6 shows that in this embodiment the bilge keel 27 becomes smoothlynarrower towards the aft body of the ship 20 and the edge 24 of thebilge keel 27 joins the narrowed, mainly vertical side 64 of the hullonly quite in the aftship.

In the FIGS. 8 ja 9 is shows the second embodiment of the ship's hull21, where the bilge keel 27 don't reach the aftship of the hull 21. Thenarrowing of the hull 21 begins from the same point 62 as in the exampleabove, but the bilge keel 27 joins the narrowed side 64 of the aft bodyalready in point 67.

Thus in narrowed area of the aft body of the ship 20 will be formed anarea, where the side of the hull 21 is mainly vertical without anyappendages directed to the side. Vertical area of the side without anyappendages plays a significant role in the loading operations of theship.

Design of the aft body of the ship's 20 hull 21 is of importance also towater flow caused by the propeller during the backing. For icebreakersit is advantageous if the water flow caused by propeller goes upwardsand washes the hull's 21 side during the backing. Thus the frictionbetween the hull and ice decreases and the movements of the ship in icebecomes easier. The washing effect can be further improved by using aknown air bubbling system, which is not described in this application.

FIG. 10 shows that the upper surface 66 of the bilge keel, which nearthe narrowed area 63 forms an angle of about 15° together with thehorizontal plane, turns to more vertical position towards the aft bodyof the hull. In the cross section point of FIG. 10 this angle is alreadyabout 60°. In other words, while the bilge keel 27 becomes narrowertowards the aft body of the ship 20, its slope changes and it graduallymerges into the mainly vertical side 64 of the aft body hull.

The smooth merging of the bilge keel 27 into the shape of the ship's 20hull 21 decreases the hull's flow resistance when the ship is movingahead, but it is useful also during the backing. This kind of designcauses effectively the water flow against the icebreaker's hull 21during the backing as described above. Shortened bilge keel according tothis embodiment is in many cases sufficient for achieving suitablestability characteristics.

FIGS. 11 and 12 show the third embodiment of the invention, where on theside of the ship's 20 hull 21 have been formed recesses 60 so thatnarrowing of the hull is formed only in the area of the water line 22.Both in the foreship and aftship the hull 21 is wider. The hull is wideralso below the water line 22, where will be formed a bilge keel 27,which is nearly as long as the whole length of the ship, and above thewater line, where is an overhang 28 of the deck 23. The overhang 28 isso big that the deck 23 is as wide as the hull 21 at the bilge keel 27.FIG. 13 shows the shape of the cross section according to thisembodiment.

It can be seen from FIG. 13 that the shape of the recesses 60 has beenadapted to the shape of the ship's hull 21 so that their upper edge 61follows the line of the deck 23 and lower edge is of arched shapeaccording to the shape of the hull 21. The arched formed of the loweredge 62 follows the arched part of the hull 21 below the recess 60 wherethus will be formed an arched bilge keel 27, which is nearly as long asthe whole length of the ship 20.

FIG. 14 shows schematically a side view of a ship 20 provided with sidestructure according to invention. It can be seen from the figure that onthe both sides of the ship's hull 21 in the area of the water line 22there are formed removable sections 30. According to invention thesesections 30 are, for example, tanks which can be attached to side of theship 20 so that in fixed position they form a part of the ship's hull21. For practical reasons the tanks 30 have been divided into smallerparts. In example shown in FIG. 14 there are three tanks on both sidesof the ship.

The shape of tanks 30 corresponds to the shape of the ship's hull 21 sothat their upper edge 31 follows the line of the deck 23 and lower edge32 has arched shape according to the shape of the hull 21. The archedshape of the lower edge 32 results in the fact that the arched part 24of the hull 21 below the tanks 30 forms an arched bilge keel when thetanks 30 are removed. This situation has been showed in FIGS. 19 and 22.

FIG. 15 shows the top view of the ship according to FIG. 14. It can beseen from the figure that when the tanks 30 are in their place attachedto the hull 21, the ship's hull form corresponds to wide hull of anicebreaker. When the tanks 30 are removed, the ship's hull becomesessentially narrower in the water line, thus a hull form suitable foropen water operations will be achieved. According to invention thismakes possible to achieve a hull form which can be designed to besuitable both for icebreaking and open water operations.

The tanks 30 decreasing the breadth of the ship's 20 hull 21 can consistof one section only or several separate removable sections. In exampleshown by FIGS. 14 and 15 three tanks 30 have been attached to each sideof the ship.

Tanks 30 preferably reach in the foreship 25 the area, where the hull 21begins to become narrower in the lower part of the tank. Thus the hull's21 foreship 25, which in the icebreaker pushes ice sidewards, consistsmainly of the solid hull. This arrangement decreases longitudinal forcesdirected towards the removable sections 30. The front edges 33 of themost forward tanks are bevelled in order to decrease the flow resistanceof the hull 21 in open water operations when the tanks 30 have beenremoved from the hull 21.

In the aft body 26 of the ship 20 the removable tanks preferably reachthe area, where their breadth in the area of the water line 22 remainssmall. Thus design of the hull 21 does not cause any additionalresistance. Naturally the hull 21 can be designed alternatively so thatthe tanks 30 end in aft body 26 to an appendage equivalent to that inthe fore body 25.

FIG. 16 shows a section taken from the FIG. 14 along the line XVI--XVI,which is an example of cross section of ship according to the invention.It can be seen from FIG. 16 that the removable tanks 30 attached to sideof the hull 21 of the ship 20 reach from the area of the upper edge ofthe ship's bilge 27 to the main deck 23. Main deck 23 has the breadth ofthe maximum breadth of the ship's hull 21 and removable side tanks 30have been attached below the overhang 28 of the deck 21.

When the removable side tanks 30 of the ship in FIG. 16 are removed, inthe ship's hull 21 will be formed appendage 24, which functions as abilge keel dampening the rolling of the ship 20. Looking from the side,profile of the appendage 24, which forms the bilge keel, can be straightor arched as shown in FIG. 14. For minimizing the moving resistance ofthe ship 20 arched form of the bilge keel 24 is advantageously to bedesigned so that it corresponds to direction of the water flow aroundthe hull 21 of the ship 20.

The overhang 28 of the deck 21 of ship 20 and the bilge keel 24 alsofunction as appendages, which form fenders when the ship is used forsupply operations. They replace in the area of removable side tanks 30the double side, which is normally required for supply ship.

FIG. 17 shows another example of cross section of the ship correspondingto FIG. 16. In this alternative main deck 23 of the ship 20 is in thearea of removable side tanks 30 narrower and the removable side tanks 30come above the water surface 22 at least the distance required foricebreaking operations. In the solution shown by the figure double siderequired by the supply ship rules can be done by means of the bulkheads29.

FIG. 18 shows a detail of cross section of the side structure of theship corresponding to invention in icebreaking situation. Thus theremovable side tanks 30 are attached to the ship's 20 hull 21.Attachment can be done many different ways by means of mechanical fixingdevices, hydraulics or by welding the removable side tanks partly to theship's hull. In the example shown by FIG. 18 the tank 30 has beenattached so that in the lower edge 32 of the tank 30 there is a lockingpart 34, which fits the corresponding hole in the ship's hull 21. Theupper edge 31 of the tank 30 has been locked by means of locking ear 35.This locking has been described in more details in FIG. 28.

FIG. 19 shows cross section of the side structure of the ship 20corresponding to FIG. 18 in open water operation. It can be seen fromthe figure that when the tank 30 has been removed, the ship's breadth inthe area of the water line 22 is essentially decreased. It results inthe fact that the rolling period of the ship decreases and also therolling acceleration essentially decreases. Furthermore the behaviour ofthe ship 20 in rough seas is settled down by the angle 24 of the hull21, which thus forms the bilge keel. After this the behaviour of theship is very much the same as that of a sea-going ship designed for openwater operations. From the water line 22 it can be noticed that loadingthe ship with bigger loads now has been possible, thus its draught hasincreased.

FIG. 20 shows schematically the movement of the removable side tank 30to the ship's 20 side and away from it. In this example the ship 20 isto be inclined so much that tank 30 can be lifted by means of a crane toits place and away from it. When fixing the tank 30 to its place it isat first to be laid on the bilge keel 24 of the ship's 20 hull 21 sothat the locking part 34 situated in the lower edge 32 of the tank 30goes into corresponding hole 41 in the hull 21. After that tank 30 canbe let press against the hull 21 and the upper edge 32 of the tank 30 isto be locked by means of the locking ear 35 to the corresponding ear 42in the hull 21. By removing the tanks 30 from the hull it is easy tocarry out their maintenance and repair in suitable conditions withoutdisturbing the ship's operations.

FIG. 21 show the second embodiment of a detail of the side structure ofthe ship 20 according to FIG. 17 in icebreaking situation. This figurediffers from the corresponding FIG. 18 in that the tank 30 is higherreaching level of the deck 23. Thus the deck 23 of the ship 20 becomesnarrower when the tank 30 is removed. FIG. 22 shows this situation whenthe ship operates in open water.

FIG. 23 shows an alternative to open water operation of FIG. 22. Here toside of the ship 20 has been attached framework 40, dimensions andattachment of which correspond to the tank 30 used in the winter.Purpose of the framework 40 is to form a fender on the ship's side. Atthe same time, deck 23 becomes also in open water operations as wide asthe max. breadth of the ship. Because structure of the framework 40 isopen, it doesn't affect rolling and heaving characteristics of the ship.

FIG. 24 shows the side structure of the ship 20 with vertical side. Herethe lower part of the tank 30 is resting on the locking part 34 and itsupper part has been locked by locking ear 35. In FIG. 25 the ship 20 iswithout removable side tank 30. This kind of structure is concerned whenno bilge keel is used in the ship.

FIG. 26 shows a section of a detail of the ship's side structure. Thestructure of removable side tank 30 is such that its bulkhead 36 andbulkhead 43 of the ship's hull 21 are on the same place. Furthermorethere are supporting pieces 37 and 44 between them.

FIG. 27 shows another detail of the ship's side structure. In ship'shull 21 there is a guide hole 45 where the corresponding quide notch 38leans on. The guide notch 38 takes the longitudinal stresses caused byattachment of the tanks 30. In icebreaking operations it isadvantageous, that water between the removable side tanks and the ship'shull freezes. This freezing equalizes the local stresses directedtowards the joints.

FIG. 28 shows a detail of the locking part of the ship's side structure.When attaching the tank 30, the locking ear 35 in the upper part of thetank 30 goes into the hole in the ship's hull 21, where is correspondinglocking ear 42. In locking situation the wedge-shaped piston 48 of thehydraulic cylinder 47 goes into holes 35 and 46 in both ears 35 and 42thus locking the tank 30 to the hull 21 of the ship 20.

FIG. 29 shows an embodiment where the upper part of the removable sidetank 30 has been attached to the hull 21 of the ship 20 by means of arail 47. Here the rail 47 has been turned onto the tank 30 and lockedinto the ear 48 on the tank 30. In FIG. 17 the removable side tank 30has been removed, thus the rail 47 has been raised up.

FIG. 31 shows the top view of the ship's hull corresponding to FIG. 15.In this embodiment the hull 21 of the ship 20 is built, however, suchthat it is mainly used without the removable side tanks 30. Ifnecessary, they can, however, be added as shown in FIG. 31.

In the embodiment shown in FIG. 32 and 33 on the hull 21 of the ship 20has been attached a separate section by means of appendage 49 and joint50. In FIG. 32 the section 30 has been turned round the joint 50 intolower position, in which case the ship 20 can be used, for example, asan icebreaker. In FIG. 33 section 30 has been turned up and the breadthof the ship 20 at the water line 22 has decreased. Characteristics ofthe ship 20 are now advantageous in open water operations.

FIG. 34 shows an embodiment wherein the separate section 30 of the hull21 of the ship 20 has been turned towards the hull 21, for example, foricebreaking situation. In next FIG. 22 section 30 has been turned roundthe joint 50 into horizontal position. Here the section 30 formstogether with the solid bilge appendage 27 in the hull an extremelyeffective bilge keel.

FIGS. 36 and 37 show an embodiment wherein the separate section 30 hasbeen attached to the hull 21 of the ship 20 into vertical guides 51. InFIG. 36 section 30 is in its lower position and the hull 21 of the ship20 at the water line 22 is wide. In FIG. 37 section 30 has been raisedup in guides 51 when the hull 21 of the ship 20 is at the water line 22narrow and below the water line is formed the bilge keel 27.

For those skilled in the art it is clear that the different embodimentsof the invention can vary within the scope of the patent claims.

We claim:
 1. A hull form of a monohull icebreaking ship wherein on thesides of the hull aft of a relatively wide foreship there is formed atleast one recess, wherein at least part of the hull is in this placenarrower than the foreship in the vicinity of the waterline, and whereindue to narrowing of the wide hull after the foreship there are sidewardsdirected steps beneath the waterline on both sides of the hull formingan extension to the foreship and serving as bilge keels to dampen therolling of the ship, wherein on the hull of the ship, on the aft side ofsaid wide foreship is a reamer connected to the recess.
 2. A hull formof an icebreaking ship according to claim 1, wherein the bilge keelspossess a length which is at least part of a length of said recess.
 3. Ahull form of a ship according to claim 2, wherein the recess and thebilge keel located below the waterline are at least in the midbody ofthe ship.
 4. A hull form of a ship according to claim 2, wherein therecess is found in the midbody of the ship and extends to the stern. 5.A hull form of a ship according to claim 2, wherein in the midbody ofthe ship the bilge keel, which is as wide as the nominal beam tapersnarrowly to the stern, and that in the aft body of the ship the bilgekeel smoothly joins the narrowed, essentially vertical side of the hull.6. A hull form of a ship according to claim 2, wherein in the midbody ofthe ship the bilge keel, which is as wide as the nominal beam, endsessentially before the stern so that towards the aftship the uppersurface of the bilge keel, which is at an angle of 0°-20° with respectto the horizontal plane, tapers to a more upright position smoothlyjoining the mainly vertical, narrowed side of the hull.
 7. A hull formof a ship according to claim 1, wherein a deck of the ship has in thearea of the recess at least in part of length of the recess mainly thesame width as in the area of the hull without narrowing so that in thisarea of the ship is a work deck with full width in spite of thenarrowing of the hull's waterline.
 8. A hull form of a monohullicebreaking ship according to claim 34, wherein stepped bilge keels arelocated in the midbody of the hull, and the sides of the narrowed atbody of the hull are without appendages and substantially vertical sothat water flow caused by a propeller washes the side during backing. 9.A hull form of a ship wherein on the sides of the hull aft of arelatively wide foreship there is formed at least one recess, wherein atleast part of the hull is in this place narrower than the foreship inthe vicinity of the waterline, and wherein due to narrowing of the widehull after the foreship there are sidewards directed steps beneath thewaterline on both sides of the hull forming an extension to the foreshipand serving as bilge keels to dampen the rolling of the ship, andwherein on the hull of the ship, on the aft side of said wide foreshipis a reamer connected to the recess, wherein the lower surface of thereamer is a sloping surface rising towards the stern of the ship, whichat the same time forms the upper surface of the recess.
 10. A hull formof a ship wherein on the sides of the hull aft of a relatively wideforeship there is formed at least one recess, wherein at least part ofthe hull is in this place narrower than the foreship in the vicinity ofthe waterline, and wherein due to narrowing of the wide hull after theforeship there are sidewards directed steps beneath the waterline onboth sides of the hull forming an extension to the foreship and servingas bilge keels to dampen the rolling of the ship, and wherein on thehull of the ship, on the aft side of said wide foreship is a reamerconnected to the recess, wherein the recess is located between the bilgekeel and reamer.
 11. A hull form of a ship wherein on the sides of thehull aft of a relatively wide foreship there is formed at least onerecess, wherein at least part of the hull is in this place narrower thanthe foreship in the vicinity of the waterline, and wherein due tonarrowing of the wide hull after the foreship there are sidewardsdirected steps beneath the centerline on both sides of the hull formingan extension to the foreship and serving as bilge keels to dampen therolling of the ship, and wherein on the hull of the ship, on the aftside of said wide foreship is a reamer connected to the recess, whereinthe recess is between the bilge keel and reamer and forms a passage,which is wider towards the stern of the ship and joins the narrower partof the hull of the ship's aft body.